1. Field of the Invention
The present invention relates to a radiation image capturing apparatus for capturing the radiation image information of a subject using a radiation dose detector for detecting a radiation dose that has passed through the subject, a method of controlling such a radiation image capturing apparatus, an apparatus for and a method of adjusting a radiation dose detector.
2. Description of the Related Art
In the medical field, there have widely been used radiation image capturing apparatus which apply a radiation emitted from a radiation source to a subject and guide the radiation that has passed through the subject to a solid-state detector to detect the radiation, for thereby recording radiation image information of the subject.
The solid-state detector includes a solid-state detecting unit comprising a matrix of charge collecting electrodes formed on an insulating substrate and a radiation conductor disposed on the charge collecting electrodes for generating electric charges depending on the radiation that is applied to the solid-state detecting unit. The electric charges generated by the radiation conductor as representing radiation image information are collected by the charge collecting electrodes and temporarily stored in an electric storage unit. The collected electric charges are converted into an electric signal, which is output from the solid-state detector.
One known radiation image capturing apparatus is a mammographic system used for breast cancer screening. The mammographic system comprises an image capturing base housing a panel-like solid-state detector for supporting a subject's breast, a presser plate disposed in confronting relation to the image capturing base for pressing the breast against the image capturing base, and a radiation source for applying a radiation through the presser plate to the breast.
The subject's breast comprises a mammary gland region and a fat region. In terms of breast cancer screening, the area of interest is the mammary gland region because it is highly susceptible to breast cancer. While the mammary gland region has a large coefficient of absorption of radiations, the fat region has a small coefficient of absorption of radiations and absorbs almost no radiations (see Japanese laid-open patent publication No. 2000-197624).
In order to acquire appropriate radiation image information of the mammary gland region, it is necessary to set suitable exposure control conditions based on the mammary gland region for applying a desired dose of radiation to the mammary gland region. The exposure control conditions include a tube voltage, a tube current, a radiation exposure time, etc. to be established for the radiation source. Of these exposure control conditions, the tube current and the radiation exposure time are the most important conditions for determining a dose of radiation to be applied to the subject. The position of the mammary gland region varies from individual to individual and differs according to various factors including body type, age, etc. The radiological technician estimates the position of the mammary gland of the subject based on the findings of the breast to be diagnosed, determines a position for a radiation dose detecting sensor with respect to the breast, and moves the radiation dose detecting sensor to the determined position. Therefore, as the experience and skill of the radiological technician plays an important role in determining a best position for the radiation dose detecting sensor, appropriate radiation image information of the mammary gland regions of various subjects may not stably be obtained. It is a complex task to move the radiation dose detecting sensor because it is manually moved by the radiological technician.
The radiation dose detecting sensor includes a light-detecting surface having a certain area. Insofar as the mammary gland region to be imaged covers the entire light-detecting surface of the radiation dose detecting sensor, a detected value from the radiation dose detecting sensor properly represents the dose of the radiation that has passed through the mammary gland region.
However, if the mammary gland region to be imaged covers only a portion of the entire light-detecting surface, then a detected value from the radiation dose detecting sensor represents the sum of the dose of the radiation that has passed through part of the mammary gland region and the dose of the radiation that has passed through part of the fat region. Consequently, the radiation dose detecting sensor produces a detected value that is different from the correct value to be detected, and fails to set an appropriate dose of radiation to be applied to the subject.